Ejection of sodium from sodium sulfide by the sputtering of the surface of Io

Abstract

Measurements have been made of the sputtering yields, the mass spectra of ejected molecules, and ejection rates for various kiloelectronvolt ions incident on sodium sulfide (Na2S). The sputtering yields were small compared to those measured earlier for the more volatile sulfur (S8) and SO2 due to the strong ionic bonding in the solid. The mechanism of sputtering for the corotating sulfur and oxygen ions in Jupiter's magnetosphere is due to a cascade of quasi-elastic collisions initiated by the incident ion. The mass spectrum indicated that sodium is ejected predominantly as a molecule with a lesser amount ejected as atomic sodium. Making several assumptions it seems unlikely that the sputtering of Na2S by magnetospheric ions can maintain the observed neutral cloud densities. Instead, the sodium probably exists as a larger polysulfide for which we show that the sputtering yield should be greater.